Power amplifiers are electronic devices that increase (i.e., amplify) the power of an electric signal. Power amplifiers are widely used in low-power communication systems. Typically, a power amplifier is located in the output stage of a transmission chain and is configured to increase the power of a radio frequency (RF) signal before it is transmitted from an antenna.
Communication systems employing power amplifiers may send signals according to certain communication standards (e.g., EDGE, WCMDA, LTE, etc.). Many such communication standards allow for different levels of output-power with different statistics of emission. Therefore, power amplifiers are often operated to generate output signals spanning a wide output power range (e.g., from a low output power to a maximum output power). However, when a power amplifier is operated to output a signal having less than the maximum output power for which the amplifier is designed, the efficiency of the power amplifier decreases.
The decrease in efficiency of a power amplifier may be mitigated by lowering the DC supply voltage at the output of the power amplifier in order to lower the overall power consumption of the power amplifier. The effectiveness of such a solution depends upon an optimum load matching between the power amplifier load and an output impedance, which is controlled by an output matching network located at the output of the power amplifier. To retain a high efficiency, the output matching network changes the output impedance when the supply voltage changes. Changing the DC supply voltage without a corresponding change in the output impedance, causes an impedance mismatch that reduces efficiency of the power amplifier.